1. Field of the Invention
This invention relates to the field of miniaturised optical sensors and more particularly sensors intended for the measurement of a fluorescent marker in small cavities, for example, the small bronchia, for the in vivo measurement of bronchial capillary permeability.
2. Description of the Related Art
The use of an interferometric heterodyne probe, integrated onto silicon, is known in the state of the technology of miniaturisable optical sensors, notably in biomedical applications.
The principle of heterodyne technology in interferometry is based on the modification of the optical frequency on one of the arms of the interferometer. The interference fringes can be modulated in phase and detected photo-electrically. The technique allows access to the phase of the phenomenon observed directly from the optical signal. In comparison with homodyning techniques, that link counting of fringes to methods of linear interpolation, heterodyne detection has two main advantages:
an improvement of the resolution linked to the phase extraction, PA1 better tolerance of low frequency noise thanks to the shift of the useful signal far form external parasite signals. PA1 "Integrated optical gas sensors using organically modified silicates as sensitive films", Sensors and Actuators B, Vol. 11, (1993), pp. 361-374, 4. A. Brandenburg, R. Edelhauser, F. Hutter, PA1 "Integrated optical sensors for halogenated and non-halogenated hydrocarbons", Sensors for Actuators B, Vol. 11,(1993), pp. 207-212, G. Gauglitz, J. Ingenhoff, PA1 "Influence of thin superstrate films on evanescent waves in surface waveguides", Ber. Bunsenges. Phys. Chem, Vol. 11, (1991), pp. 1588-1563, G. Gauglitz, J. Ingenhoff, PA1 "Integrated optical chemical and direct biochemical sensors", Sensors and Actuators B, Vol. 29, (1995), pp. 37-50. PA1 "Integrated optics with macro-flow cell", Proc. SPIE, Vol. 1793, (1992), pp. 199-211, A. A. Boiarski, J. R. Busch, B. S. Bhullar, R. W. Ridgway, V. E. Wood.
The transposition of the massive architecture of heterodyne interferometry into a fibre version leads, in general, to a degradation of the signal to noise ratio, linked to external interference and parasitic reflections. In addition, "all fibre" shift devices are rare and one comes up against problems of coupling the massive phase modulators and the input of the fibre that seriously limit the applications that require miniaturisation of the probe.
Thanks to the optical technologies for integration onto silicon, the future is assured for micro-interferometers of very small size that can be manufactured by monolithic integration. The optical device integrated onto silicon is particularly well suited to "biomedical sensor" applications where a sensitive "superstrate" layer of hydrocarbon or polymer is deposited on the measurement arm of the interferometer (generally the Mach-Zehnder configuration). Some typical examples of biological and chemical sensors are described in the following documents:
If numerous integrated optical "bio-sensors" have appeared, they are based principally on homodyning technologies. The technique of integration of optical micro-probes comes up against the difficulty of producing a modulation of the reference beam necessary for heterodyning onto a silicon substrate that is playing a passive role.